It is well known that a visually cognizable image can be created by placing many individual picture elements (i.e. pixels) in a proper arrangement. For example, video displays create images in this manner, and images which are included in digital data transmissions are recreated using pixels. It is also the case that images which are created using nuclear magnetic resonance techniques (MRI), must be collated from data as pixels.
For purposes of discussion, in order to better appreciate the notion of pixels in the context of the present invention, consider a substantially rectangular, two dimensional image in an x-y plane. Further, consider that the image, or picture, is divided into an "m" number of segments in the x direction and into an "n" number of segments in the y direction. In this arrangement, the image would comprise m.times.n pixels. As is well known, and easily demonstrated, if all of the pixels in the image are sufficiently small the human eye is incapable of distinguishing individual pixels. Instead, due to the small size of the individual pixels, they blend together into a coherent continuous image. Thus, the total image, rather than individual pixels, is perceived.
As implied above, images that are created using MRI techniques will comprise an arrangement that includes a large number of potentially individually different pixels. Due to the nature of MRI, however, the process of creating individual pixels for an MRI image requires some means for distinguishing each of the pixels from all of the other pixels in the image. To make these distinctions, MRI techniques typically include some scheme for encoding the pixels so that they may be subsequently recaptured to create the desired image.
In order to consider the specifics of an encoding scheme, it is first to be appreciated that, in an MRI procedure, the in situ biological tissue to be imaged is irradiated with radio frequency (RF) signals. Importantly, these signals have a particular frequency which is dependent on the magnetic field strength in the location of the tissue that is to be imaged. This frequency is technically referred to as the Larmor frequency. As is well known in the art, irradiation of tissue at the Larmor frequency causes nuclei of the tissue to generate spin echo signals. For purposes of MRI, these spin echo signals are recordable. The recorded spin echo signals, however, must be processed in order to obtain a cognizable image.
One of the problems to be overcome when processing MRI signals stems from the fact that all of the in situ biological tissue that is to be imaged, is irradiated simultaneously. Accordingly, each irradiation results in a measurement which includes the spin echo signals from all of the nuclei in the tissue. It happens that in order to generate m.times.n pixels for an image, m.times.n encodings must be accomplished. Without encoding, however, all responses to the plurality of irradiation epochs would be the same and there would be no way to distinguish the response of nuclei in one portion of the tissue from the response of nuclei in other portions of the tissue. Thus, encoding is required.
The method envisioned by the present invention for encoding pixels of an MRI image employs a two dimensional phase encoding scheme. For phase encoding, each encode signal is characterized by a distinctive two dimensional spatial pattern of phase. By changing the phase from one encode signal to another for each irradiation of the imaged tissue, distinctions can be made in the spin echo signals that are generated. These distinctions will then be useful in sorting out and distinguishing the various pixels during recreation of the image.
In addition to the technical aspects of phase encoding, it is also necessary to consider when the phase encoding should be accomplished in an MRI procedure. For remotely positioned MRI devices which employ substantially inhomogeneous magnetic fields, specific techniques are involved, and specific time sequences for encoding need to be observed. U.S. Pat. No. 5,304,930, which issued to Crowley et al. for an invention entitled "Remotely Positioned MRI System", and which is assigned to the same assignee as the present invention, discloses methods for creating images using inhomogeneous magnetic fields.
Briefly, the '930 patent discloses and claims a remotely positionable MRI device which includes a magnet for generating a static inhomogeneous magnetic field that is external to the magnet. Importantly, the magnet is configured to establish a region in the magnetic field which can be effectively used as a substantially flat measurement surface. Further, the measurement surface is characterized by the condition that the magnitude of the field strength (B.sub.o) is substantially constant, and the field gradient in the direction normal to the measurement surface (G.sub.z) is approximately.
In accordance with the teachings of the '930 patent, an RF antenna is incorporated into the device to irradiate the nuclei in the measurement surface with a tilting pulse at the appropriate Larmor frequency. In a manner well known in the art, this first pulse tilts nuclei out of the spin orientation they had assumed under the influence of B.sub.o in the static magnetic field. Once tilted, the nuclei can be encoded. In order to encode the tilted nuclei, a first gradient coil and a second gradient coil are incorporated into the device and are used for changing magnetic field gradients in the measurement surface in both the x (G.sub.x) and y (G.sub.y) directions. More specifically, after the nuclei have been initially tilted by the first radiation pulse from the RF antenna, the first and second gradient coils can be selectively activated to orient the spin vectors of individual nuclei in the x-y plane of the measurement surface. This encodes the nuclei with a predetermined transverse phase pattern.
Another function of the RF antenna of the device disclosed in the '930 patent is to refocus the nuclei after they have been encoded. It happens that due to the permanent field gradient in a inhomogeneous magnetic field, nuclei within the gradient precess at different rates. This results in both coherent defocusing and incoherent diffusion of the nuclei which require that the nuclei be periodically refocussed at an accelerated rate. This refocussing is accomplished by transmitting a refocussing pulse at the appropriate Larmor frequency from the RF antenna. Importantly, accelerated refocussing, when combined with transverse gradient applications, and signal averaging results in the generation of encoded spin echo signals from the nuclei that are useable in image reconstruction.
The MRI device of the invention disclosed in the '930 patent also includes electronic means for coordinating the operations of the RF antenna (tilting and refocussing) with the operation of the encoding gradient coils. Specifically, the transmission of appropriate RF irradiation pulses from the antenna, the encoding of the nuclei by the first and second gradient coils, and the reception of encoded spin echo signals from the nuclei must occur in a predetermined and controlled sequence. Additionally, electronic means are required for converting a plurality of encoded spin echo signals into encoded NMR responses and, in turn, for converting a plurality of NMR responses into an image.
Further to the brief summary of the disclosure of the '930 patent presented above, the entire disclosure of the '930 patent is fully incorporated herein by reference. Additionally, it is to be appreciated that the systems and methods disclosed in the '930 patent as well as those for the present invention may be used in other systems that have inhomogeneous magnetic fields.
Superposed on the above discussion for MRI signal processing is the fact that the constant artifacts of noise potentially corrupt the finally created MRI image. Specifically, in the context of inhomogeneous field systems, the acoustic and electromagnetic remnants resulting from the accelerated RF irradiation pulses may present artifacts during each step of data recording. Unlike thermal noise, the aforementioned artifacts may be coherent, and hence, cannot be attenuated by averaging techniques. Furthermore, for phase encoding, it is known that pixels of the image which have been subjected to the least amount of phase change during encoding are more susceptible to noise corruption than are those pixels whose encoding phase changes have been more pronounced. Since a phase encoding is typically accomplished by varying the number of cycles of phase change along the entire field of view, it happens that the pixels at the center of the length of pixels will experience little, if any, encoding phase changes throughout the entire MRI procedure. On the other hand, those pixels which are farthest from the center will experience extensive changes in their encode phase from one encode to the next encode. The result is that the pixels at the center of the reconstructed image are most affected by noise. As a practical matter, this means that the center of the image is corrupted by noise, and that the corners of the image have the least artifact.
In light of the above, it is an object of the present invention to provide a method for suppressing acoustic noise from an MRI image which establishes the clearest resolution for the image at the center of the image. Another object of the present invention is to provide a method for suppressing noise from an MRI image which effectively filters constant artifacts, such as exogenous noise, from portions of an MRI image which require the best image resolution. Still another object of the present invention is to provide a method for suppressing noise from an MRI image which is relatively simple to implement and comparatively cost effective.